As for the alternative energy to replace the fossil fuel energy such as petroleum and natural gas, studies are being made at present on coal energy, biomass energy, nuclear energy, and natural energy such as wind energy and solar energy.
In the case of using coal energy as the alternative energy, a large amount of carbon dioxide is released by the combustion of coal and this is thought to become a problem. For solving this problem, it has been proposed to collect carbon dioxide at the combustion of coal and store the collected carbon dioxide underground, and numerous research projects are being carried out regarding this matter. However, the long-term stable storage of carbon dioxide is not certain and also, the places suitable for storage are unevenly distributed. Furthermore, the high cost required for the recovery and transfer of carbon dioxide and the injection of carbon dioxide into the ground will become a problem. In addition, the possibility that the combustion of coal will raise an environmental issue due to generation of sulfur oxide (SOx), smoke and the like will also become a problem.
The biomass energy as the alternative energy, particularly the biofuel mainly comprising ethanol, is attracting a great deal of attention. However, a large amount of energy is necessary for the production and concentration of ethanol from plants, and this is sometimes disadvantageous from the viewpoint of energy efficiency. Furthermore, in the case of using corn, soybean, sugarcane or the like as the raw material for biofuel, since these are of course used as food and feed, escalation in the price of food and feed is incurred. Accordingly, the biomass cannot be considered as a substantial energy source except for special regions such as Brazil.
Use of nuclear energy as the alternative energy source is not expected to make great and worldwide progress, because no satisfactory solution is found for the treatment of radioactive waste from nuclear power plants and there are many opposing opinions based on the fear of nuclear proliferation. Instead, use of nuclear energy as the alternative energy will decrease in the long term with an increase in the abolishment of aging nuclear reactors.
As described above, all of the coal energy, biomass energy and nuclear energy cannot be said to succeed in solving the problems of sustainability and carbon dioxide generation giving rise to global warming. Consequently, the natural energy such as wind energy and solar energy is considered to be an ideal energy source.
With respect to the use of wind energy as the alternative energy, wind-power-generation plants are recently spreading around the world. However, the suitable places having stable wind and no danger of typhoon, hurricane, thunderbolt or the like, or where the noise generated from a windmill does not become a problem, is limited. Accordingly, wind energy is insufficient by itself, though it is a strong candidate for alternative energy.
Solar energy is believed to be a most stable and intensive natural energy as the alternative energy. Particularly, there are vast deserts near the equator called the Sun Belt of the globe, and the solar energy there is almost inexhaustible. In this respect, it is assumed that energy as much as 7,000 GW can be obtained by the use of a few percent of the area of the deserts extending in the southwestern area of the United States, and that all of the energy for all human beings can be supplied by the use of only a few percent of the area of the desserts in the Arabian Peninsula and North Africa.
In this way, the solar energy is very potent as the alternative energy, however, from a practical use standpoint, it is considered necessary to solve the problems that (1) the energy density of solar energy is low and (2) the storage and transfer of solar energy are difficult.
As for the problem that the energy density of solar energy is low, a resolution by collecting solar energy by means of a massive collector has been proposed. However, the storage and transport of solar energy are very difficult in particular when the transport distance is long and the amount of energy is large.
Solar energy is generally converted into electric power as the secondary energy directly by a solar cell or indirectly by a steam turbine or the like, and thereby turned into a form convenient for use and transport. When solar energy is converted into electric power, the electric power energy can be transferred on an electric power transmission line, and therefore the problem of energy transfer is overcome in principle. However, in the case where a plant for obtaining electric power energy from solar energy is installed in a solar energy-rich desert region, a high-capacity electric power transmission line needs to be newly built and maintained, but this is difficult in many cases. Furthermore, it is thought very difficult to transfer the electric power energy obtained from solar energy, for example at a plant in a desert region to another continent or island country across the ocean.
Storage of the electric power sometimes becomes a problem. Development of a battery for storing electric power is a previously-existing major theme and is being continued all over the world. However, even the most-advanced lithium ion battery is not satisfactory with regard to the storage of a large amount of electric power, and a battery particularly for a large amount of electric power needs to be developed in terms of safety. Also, in the plant for obtaining electric power energy from solar energy, a massive thermal storage unit, an auxiliary boiler and the like, as well as the battery, are required in case power generation becomes difficult due to bad whether or the like, and these constitute a huge construction cost.
Studies are being also made to convert solar energy as the primary energy into hydrogen as the secondary energy, and synthesize ammonia, methane or the like by using the obtained hydrogen as a raw material (see, Patent Document 1).
Hydrogen is attracting attention as clean energy, but similar to electric power, its storage is a major problem. For the supply to a fuel cell, much research on hydrogen storage have been recently carried out, and it is becoming apparent that the practical application thereof is not easy. Also, as for the transfer of hydrogen, the construction of hydrogen pipelines is more difficult than the construction of electric power transmission lines. In particular, the construction of a hydrogen pipeline network infrastructure for the supply to users is difficult. Furthermore, liquid hydrogen must be stored at −253° C., and therefore storage of liquid hydrogen can not be considered at present, except for special usages such as space development.    [Patent Document 1] Japanese Unexamined Patent Publication No. 2006-319291